1. Field of the Invention
The present invention relates to a washable filter for purifying water.
2. Description of the Related Art
For filtering water filters are used in which the turbid water is led in a downward or upward direction through a filtration zone filled with filter material. Sand, gravel or also activated carbon are used as filter materials. Usually such filters are periodically cleaned by means of so-called backwashing. That means, rinsing liquid is sent in reverse direction to the filtration flow through the filter material in order to liberate the latter from sludge.
Examples of such arrangements are described in U.S. Pat. Nos. 3,886,071, 4,246,119, 3,111,487, 3,239,061, DE-1033 184, DE-11 83 050, DE 3245 093 and DE 19 62 881. All these arrangements deal with filters in which the water to be purified is sent through a filter layer from top to bottom during the filtration. In order to clean the filter layer, rinsing liquid is again sent in reverse direction, i.e. from bottom to top, through the filter layer.
Such arrangements have considerable disadvantages. The sludge accumulating in a sedimentation chamber, which is arranged underneath the filtration layer, can only be removed with difficulty by means of this process. This is because the sludge must be forced through the entire filter bed at first. That means that a considerable pressure and/or expenditure of time is necessary when cleaning. Moreover, the filtration cannot be continued during the desludging process.
Furthermore such backwash filters are not suitable for the construction of several filter layers with different grain sizes. This is because the fine grain is taken upwardly during the backwash process while the coarse grain is progressively moved to the lower layers. After several rinsings, there is an outer layer on the top of the filter bed which consists of very fine grains and clogs very quickly during subsequent filtration. Consequently, the entire remaining filtration layer under the fine grain layer hardly has any effect.
A further disadvantage of the reverse flow washing lies in the fact that the arrangement of the microorganism flora within the filter layer is destroyed to a large extent. During the filtration process, an accumulation of microorganisms of different types is formed in dependence on the contaminant composition of the water which is changed under the influence of these microorganisms. It follows that the microorganism population is changed with progressive filtration distance and thus the different contaminant fractions are gradually removed from the water. In this connection, the oxygen gradient being formed also has a considerable influence. Thus, a high oxygenation takes place in the upper parts where the raw water is supplied while the oxygen content is lower in the lower layers. For this reason, preferably microaerophilic microorganisms settle in these layers. Reverse flow washing destroys the order of this microorganism flora. It follows that with the beginning of each new filtration phase a certain preparatory stage has to pass until the microorganisms have found their order again.
From U.S. Pat. No. 3,534,855, DE 15 36 867 and DE 28 48 660 processes are known in which the liquid to be cleaned is forced through the filter from bottom to top and rinsing occurs from top to bottom. It is true that these processes have the advantage that sludge possibly settling in the sedimentation chamber below the filtration layer is drawn off by the outlet arranged below. However, the further disadvantages of the reverse flow washing as mentioned above remain in existence to a large extent. In addition, the outlet positioned below is not suitable for filter arrangements which are to be embedded in the soil as this is the case with pond filters, for instance. Piping within the soil requires additional construction and service expenses. On the other hand, a direct outlet of the activated sludge into the soil is not recommendable, because this process would quickly lead to a poisoning and clogging of the soil surrounding the filter.
Finally, from DE-13 55 550 a device for filtering water is known in which water is applied on the filter material in the direction of filtration in order to wash. For this, a water supply pipe is arranged above the filtration layer. At the end of this pipe, rotatable spray arms are positioned. During the washing process water is now applied from these spray arms onto the filtration layer. In doing so, the spray arms are permanently rotated and slowly driven into the filter material at the same time.
It is true that this filter system does not have the above-mentioned disadvantages when washing. But the slow rotation of the spray arms only works if filter sand is used. However, even then there is the risk that the nozzles of the spray arms will gradually clog. If other materials are used, such as coarse-grained gravel, the spray arms can only be moved with difficulty. Moreover there is the risk that the spray arms will be damaged. When using sensitive filter materials, such as porous clay, there is further the danger that these materials clog and the desired filtration effect is lost. Furthermore the construction of such a rotatable device requires quite a constructional expenditure. In addition to this, the energy expenditure necessary for the rotation needs to be considered.
Finally, also this system results in a destruction of the arrangement of the microorganism flora. Due to the permanent rotation of the spray arms into the filtration layer, the latter is constantly agitated. Therefore, each beginning of a filtration cycle requires a certain starting time until the microorganism population has found its order again.
DE 32 14 419 describes an arrangement consisting of a closed vessel which is equipped with filter devices. At first, the raw material is conveyed into a filter preliminary chamber via a supply pipe arranged in an upper region. The filter preliminary chamber surrounds a filter bed which is separated from the chamber by a perforated layer. The water from the filter preliminary chamber enters the filter layer at the side. The filtered water is drained via a pipe arranged below. Additional ventilation devices are provided in the lower part of the filter preliminary chamber for constantly mixing the water in the chamber.
Considerable disadvantages result from the design of this arrangement. Thus, the filtration capacity and the filtration result respectively are very unsteady, since the filtering time in the filter layer depends on where the water to be filtered enters the perforated layer. The water entering the lower part has to cover a considerably shorter distance to reach the drain pipe than the water entering the upper part.
Moreover, due to the constant turbulence caused by the ventilation devices in the form of air nozzles, a sedimentation of the sludge is not possible to a sufficient extent before the water enters the filter layer. Consequently, a large number of the sludge particles from the preliminary filter directly reach the filter layer and thus contribute to a shorter service life.
Finally, it remains to be noted that the sludge drawoff is diposed in the lower part of the arrangement. The same applies to the drain pipe and the supply pipe for backwash water. For this reason, the arrangement cannot be embedded in soil. Additionally, with the backwash pipes positioned below, a cleaning of the filter layer can only be achieved in reverse direction to the filtration flow. This means again that it is not possible to clean the arrangement during running operation.
DE 31 24 556 discloses a special filter for purifying waste water contaminated by light liquids. In principle, the reference deals with a separating device which is only filled with a filter mass. Here, the following processes take place in the same vessel: on the one hand, gravity separation and the storage of the separated light liquid and, on the other hand, coalescence filtering. Thus, in this case, the vessel does not consist of two filter chambers. The liquid to be cleaned is rather led via a supply pipe positioned below and an ascending pipe to a filter layer, in which the liquid flows from top to bottom. From a chamber positioned below, the water then reaches a special drain chamber.
However, the effect of carrying out a filtering with interposed sedimentation is not achieved with this arrangement. It is rather the purpose of the arrangement to collect light liquid on the surface in the upper part of the device. During continued operation of the device, a light liquid layer growing thicker and thicker collects on the surface which extends into the filter filling. It follows that light liquid reaches the drain chamber. At that moment, however, a float provides for a closure of the drain.
The cleaning of the arrangement necessary from then on is effected by means of backwash pipes positioned below; i.e. also in this arrangement, cleaning is carried out in reverse flow. Thus, no indication can be taken from DE 31 24 556 that a biological cleaning arrangement can be operated with this system and that in doing so, cleaning can be achieved in parallel flow during running operation. Apart from that, this arrangement can also not be embedded in soil since the supply pipe and the backwash pipes are mounted in the lower region.
Finally German utility model 7816518 relates to a dripper being composed of only one filter chamber. In this case, the water supplied via a pipe from below is distributed on the surface of a filter layer by means of nozzles. The water flows through a filter layer from top to bottom and is finally used again via another pipe of the arrangement.
Thus, the principle of cleaning by means of a filter layer with interposed sedimentation cannot be taken from this arrangement. Rather, from the manner in which the removal of the filter mass can be facilitated, it is clear that this publication starts from the fact that cleaning is not possible during running operation.